1. Field of the Invention
The present invention relates to an electrode active material, an electrode having the active material, and a method for producing the active material.
2. Description of the Related Art
A currently available energy storage device includes an electrochemical double layer capacitor, a hybrid capacitor, a lithium secondary battery, a solar cell, a fuel cell or the like. More specifically, the energy storage device includes a lithium secondary battery or an electrochemical double layer capacitor (EDLC) typically called a supercapacitor.
A lithium secondary battery has an advantage of a high energy density of 20˜120 Wh/kg but has disadvantages of a low power density of 50˜250 W/kg and a low cycle life of about 500 times.
An EDLC can be rapidly charged or discharged and has a strong over-charging and/or over-discharging characteristic. Further, the EDLC can provide an extended service life span and can be employed in a wide range of temperatures, since it does not involve chemical reactions. Furthermore, the EDLC is environmentally friendly since it does not contain heavy metals. That is, since the EDLC has a variety of characteristics that no battery provides, it has been widely employed in a memory back-up power supply and the like.
Recently, a high-capacity model has been rapidly developed and an application to a high performance energy (storage) device has also been attempted. Thus, a variety of applications to a power storage system with solar cells and fuel cells, an auxiliary power source for hybrid electric vehicle (HEV) and the like have been investigated or considered.
The lithium battery and the EDLC are very similar in view of their unit cell structures and operating principles, but are different from each other in view of their charge storage mechanisms. That is, in case of the lithium secondary battery, since electrons and ions are transferred into the bulk of electrode material and the transfer relies on a Faradaic reaction, a phase transformation of an electrode material is involved. On the other hand, in an EDLC, such a Faradaic reaction is not involved (i.e., non-Faradaic process), a charging or discharging reaction is produced only at an interface (electrochemical double layer) of electrode and electrolyte, without any phase transformation of the active material.
In the meantime, a hybrid capacitor has been proposed to compensate for the disadvantages of the lithium secondary battery and EDLC.
An EDLC is configured in such a manner that a pair of positive and negative polarizing electrodes each formed of active carbon are disposed in an electrolyte containing electrolytic ions in a state where a partition membrane is interposed between the electrodes. If a direct current is applied to the electrodes, anions in an electrolytic solution move toward the positive electrode and cations move toward the negative electrode while the voltage is increased. These anions and cations form an electrochemical double layer at the interface between the electrode and the solution, which in turn is used as electric energy.
The conventional EDLC has superior power density but poor energy density. In order to apply the EDLC to energy devices, it is necessary to develop a capacitor with larger capacitance. In order to increase the capacitance of an EDLC, it is indispensable to develop an electrode material capable of forming a lot of electrochemical double layers.
Therefore, in order to form a lot of electrochemical double layers, the use of active carbon having a large specific surface area has been considered. However, the active carbon with a larger specific surface area has excellent specific capacitance per mass (F/g) but causes the decrease in the electrode density. As a result, there is a limitation in improving the specific capacitance per volume (F/ml).
An alkaline activation method of carbonizing a soft or graphitizing carbon material and then activating the carbon material together with caustic alkali at a high temperature has been proposed to increase capacitance per volume. For example, if a soft carbon material serving as a polarizing electrode material is activated together with caustic alkali such as KOH, CsOH and RbOH at a temperature of 600˜800° C. under an inert atmosphere, it is possible to manufacture non-porous carbon having a fine grain size similar to graphite and a interlayer distance d002 of 0.360˜0.380 nm. This non-porous carbon has a low specific surface area of 270 m2/g or less. However, when it is used as an electrode material of EDLC, the non-porous carbon exhibits a high capacitance per volume of 30 F/ml or more. It is believed to be due to the intercalation of electrolytic ions including solvent between layers of graphite-like fine grains.
However, the active carbon produced by the above alkaline activation process has a problem in that charge/discharge cycle characteristics are degraded by means of the expansion or cracking of electrodes due to the repeated intercalation and deintercalation of electrolytic ions into the interlayer, gas generation and the like, and the distortion of a container material as the charging/discharging process is repeated after forming an electrode of EDLC.